1. Field of the Invention
This invention relates to a torque detecting device of the magnetostriction type used for detecting torque applied on a measuring shaft.
2. Description of the Prior Art
For example, there has been such a torque detecting device of the magnetostriction type as shown in FIG. 1 to FIG. 3 (Specification and Drawings of Japanese Patent Application No. 63-50820).
The torque detecting device 1 shown in FIG. 1 to FIG. 3 is provided with a measuring shaft 2 at the center thereof. On the surface of said measuring shaft 2, concave parts 3a, 3b and convex parts 4a, 4b making a certain angle with the axial direction of said measuring shaft 2 are formed to be integral with said measuring shaft 2 at appropriate spaces, and shaped-magnetic anisotropy is given by these concave parts 3a, 3b and convex parts 4a, 4b.
In this case, said concave part 3a, convex part 4a on the one side and concave part 3b, convex part 4b on the other side are provided as a pair with an inclination of 45.degree. relative to the axial direction in opposite directions relative to each other.
The torque detecting device 1 has a pair of coils 5a, 5b disposed in facing relation to the concave part 3a, convex part 4a on the one side and concave part 3b, convex part 4b on the other side which are formed on said measuring shaft 2 as an addition to the said measuring shaft 2, and has the structure providing a cylindrical yoke 7 made from a material with high permeability at the outside of said coils 5a, 5b while leaving a gap 6 between the yoke 7 and the measuring shaft 2. In this case, the coils 5a and 5b are used for excitation means for forming a magnetic circuit wherein the measuring shaft 2 is a part of said magnetic circuit in common with detection means for detecting a magnetostriction passing through said measuring shaft 2.
In the torque detecting device 1 having such a structure, the coils 5a and 5b compose the bridge circuit in combination with resistors 11 and 12 as exemplified in FIG. 2. Said bridge circuit is provided with a variable resistance 13 for balancing and the excitative directions are adjusted into the same direction by connecting an excitative oscillator 14 to connecting points A, C in the bridge circuit. A differential amplifier 15 is connected to connecting points B, B' so as to pick up a detection output from output terminals 16, 17.
The operation of the torque detecting device 1 shown in FIG. 1 connected to the electric circuit shown in FIG. 2 will be described as follows.
In the first place, during operation AC with constant amplitude (V) and frequency (f) is applied to the coils 5a and 5b from the excitative oscillator 14. In consequence of this electric charging, lines of magnetic force forming the magnetic circuit comprised of measuring shaft 2.fwdarw.gap 6.fwdarw.yoke 7.fwdarw.gap 6.fwdarw.measuring shaft 2 are generated surrounding the coils 5a and 5b.
The lines of magnetic force flow in a part of surface of the measuring shaft 2, in particular in the convex parts 4a, 4b mainly, because the concave parts 3a, 3b are formed on the measuring shaft 2 at a certain angle with the axial direction of the said measuring shaft 2, they become magnetic reluctance. Hence the effect of shaped-magnetic anisotropy caused by said concave parts 3a, 3b and convex parts 4a, 4b is produced.
If torque is applied on the measuring shaft 2 in the direction T shown in FIG. 1, the maximum tensile stress --.sigma. acts on it because the convex part 4a on the one side is formed in the direction of 45.degree. to the right side on the contrary, the maximum compressive stress --.sigma. acts on it because the convex part 4b on the other side is formed in the direction of 45.degree. to the left side.
Therefore, if the measuring shaft 2 has a positive magnetostriction effect, the permeability of the convex part 4a on the one side increases in comparison with it when the torque is zero. On the contrary the permeability of the convex part 4b on the other side decreases in comparison with it when the torque is zero.
Consequently, increasing the inductance L.sub.1 of the coil 5a on the one side and, decreasing the inductance L.sub.2 of the coil 5b on the other side, the balance of the bridge circuit shown in FIG. 2 breaks and an output corresponding to the torque is generated between the output terminals 16 and 17.
In case the torque is applied on the measuring shaft 2 in the opposite direction, according to an adverse effect, decreasing the inductance L.sub.1 of the coil 5a on the one side and, increasing the inductance L.sub.2 of the coil 5b on the other side, the balance of the bridge circuit shown in FIG. 2 breaks and the detection output having a characteristic shown in FIG. 3 corresponding to the torque is generated between the output terminals 16 and 17.
In the torque detecting device 1 having such a structure, because the concave parts 3a, 3b and the convex parts 4a, 4b are formed in a pair with reverse inclination facing the coils 5a and 5b respectively, and the difference of magnetism variation at said concave parts 3a, 3b and convex parts 4a, 4b is detected by means of a bridge circuit, the temperature of the measuring shaft 2 changes uniformly on the whole, the zero point of the output can cease to shift and it is possible to heighten the detecting accuracy of the torque even if the permeability of the measuring shaft 2 is varied on the whole according to the temperature.
However, in such the conventional torque detecting device 1 of the magnetostriction type, if a temperature gradient is produced in the axial direction of the measuring shaft 2 owing to heat generated by a motor connected on the one side of the measuring shaft 2 for example, the permeability of the measuring shaft 2 is varied in the axial direction and the inductance L.sub.1 and L.sub.2 of the coils 5a and 5b change respectively. Accordingly, there is a problem in that the zero point of output may shift owing to the unbalance produced between the connecting points B and B' shown in FIG. 2, and the remarkable drift of the detection output sometimes occurs.